Should the Perseverance rover find it, this time there will be no premature celebration: The evidence will come back to Earth for examination first. And so this search for life comes with a twist. The mission is designed to look for signs of only ancient microbes—extinct, fossilized, hollowed out—not present life, squirming away at this very moment.
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Finding live aliens, even microbial ones, would force NASA to confront questions about interference and contamination; ancient life has the advantage of being both more likely to exist and inert. “Our best chance of identifying life on Mars would be going into the past,” says Lori Glaze, the director of NASA’s planetary-science division. Such a discovery would certainly be bittersweet, even depressing. Our corner of the solar system would seem less lonely, but the only neighbors we spotted would have turned out the lights long before we started knocking on their door.
Several billion years ago, Mars was likely warm and sloshing with water. Orbiting spacecraft have found the imprints of dried-up waterways; the Curiosity rover, America’s previous emissary, dropped into a crater and found evidence of an ancient lake, with traces of organic molecules hidden in its soil. Perhaps microbes proliferated in that muggy environment, as they did on Earth.
Today, Mars is not a pleasant place, at least not for life as we know it. It is cold and dry. With a wispy atmosphere and no magnetic field, its surface is zapped by radiation.
The Perseverance rover will land inside a crater called Jezero, where, between 3 billion and 4 billion years ago, a river flowed into a lake the size of Lake Tahoe. On Earth, the geological record of the planet’s earliest days has been crumpled under shifting tectonic plates and sandpapered by weather. On Mars, including in Jezero, it is well preserved, pristine even, ready for a rover to come by and poke at it, searching with its robotic eyes for traces of dead microbes in the mud.
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Perseverance will drill into rocks that seem promising, seal the soil it collects in little tubes, and deposit the samples on Mars’s surface. In the coming years, various machines are expected to descend on the planet to facilitate the cosmic relay race that comes next. In 2028, another rover will fetch the tubes and transfer them to a nearby rocket, which will launch to an orbiter circling overhead, becoming the first rocket to lift off another planet. The orbiter will grab the samples and head back to Earth, where it will drop them through the atmosphere and eventually into the labs of eager scientists. If this ambitious sequence goes as planned, NASA, together with the European Space Agency, will have transported martian dirt to Earth for the first time.